The underperforming defroster lines are damaged. The ones that are not working at all have been damaged to the point that they do not conduct electricity. This is most likely because the lines have been rubbed hard enough to scrape off some of the material that makes up the lines. More voltage will not fix this problem. I believe that there are kits sold at auto parts stores that claim to "fix" broken defrosters. They use a paint-like substance (that is conductive when dry) that you apply onto the damaged areas of a defroster's lines. Aside from replacing the whole glass panel with a defroster grid that is in better shape, this is your only fix. Don't try to solder the lines with regular solder at their broken areas. You'll risk breaking the glass.